Resistance 3

This will likely be the first trophy you get. The first place you can get this trophy is right at the start of Chapter 3: Haven Outskirts, you will be in a building without walls, and a dropship will come and drop off 7 or 8 Chimera. Simply throw a shrapnel grenade into the mob, which will more than likely kill 5 or 6 of them. After that, your trophy will unlock. There will be untold amounts of opportunities to get this trophy, so don't worry if you miss it here.

Note that this says "Military" chimera, meaning it will not work if you kill 3 Grims, or Leapers. The must be Hybrids, Longlegs, Steelheads, or Ravagers.

For a visual example, check out this video courtesy of sunnygrg: